The design of electrically addressed spatial light modulators (SLM) has been influenced to a great extent by the displays industry, although there are major differences between the requirements for good displays and those for SLMs. Typically a conventional liquid crystal display as illustrated in FIG. 1 and which may form the basis of an SLM, comprises a silicon substrate 1, an FET comprised by source and drain regions 2, 3 and a gate 4 in a silicon dioxide layer 5 which is apertured to provide electrical contact between a transparent electrode 6 and drain 3. Another transparent electrode 7 is provided on a transparent panel 8. A liquid crystal, for example a ferro-electric effect chiral smectic liquid crystal material 9 is disposed between the electrodes 6 and 7. The cell (pixel) may be operated by applying a control signal to the gate such that the electrode 6 is driven to a positive or negative voltage relative to the other electrode 7 so as to switch the ferro-electric liquid crystal material between its two stable states, i.e. cause it to appear light or dark, for example. Attention is directed to our GB patents 2149555B, 2149176B, 2166256B and 2188742B, which describe various aspects of the structure and operation of ferro-electric liquid crystal displays.
A second important electro-optic effect in chiral smectic liquid crystals is the "electroclinic effect" which uses the "soft mode" dielectric response (Bahr C. H. and Heppke G, Liq. cryst, 2(6), 825-831, 1981; Collings N., Crossland W. A., Chittick R. C. and Bone M. F., Proc. SPIE, 963, (in print), 1989). This electroclinic effect is capable of a sub-microsecond response as is the ferro-electric effect, but the electroclinic effect also gives an analogue response to the voltage, that is the optic axis of the liquid crystal medium rotates (in plane of the liquid crystal layer) by an amount that is proportional to the applied voltage. Switching times of several hundred nanoseconds have been observed (Davey A. B. and Crossland W. A., Second International Conference on Ferro-electric Liquid Crystals, Goteberg, Sweden, June 1989) and also significant optical responses at voltages compatible with silicon integrated circuits.
In principle, electro-optic effects in chiral smectic liquid crystals may alter the amplitude and/or phase of incident light.
The electro-optic effects in ferro-electric liquid crystals (tilted chiral smectic phases, such as Sm C*) utilise the Goldstone mode dielectric relaxation which in principle allows phase modulation of light beams as well as intensity modulation (Tomas Carlson, Bostjar Zeks, Cene Filipic and Adrijan Levslik, Ferro-electric, 1988, Vol 84, pp 223-240). This arises because the n director of the liquid crystal, which coincides with the optic axis, is not constrained to move only in a plane orthogonal to the light beam. It can adopt any position on a cone of angles. This out of plane movement is often suppressed in the realisation of electro-optic effects, for example in so-called "surface stabilised" devices (N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett. 36, 899, 1980). This, however, need not necessarily be the case (W. A. Crossland, M. Bone, P. W. Ross, Proc. S.I.D., Vol. 29, No. 3, pp 237-244, 1988).
The electroclinic effect in chiral smectic A liquid crystals utilises the soft mode dielectric relaxation, which in the device geometry normally used (Bahr. C. H. and Heppke--see above) does not allow analogue modulation of the phase of a normally incident light beam, because the principal optic axis is rotated by electric fields only in a plane orthogonal to the light beam.
For both "stabilised" Goldstone mode devices (ferro-electric) or electroclinic (soft mode) devices, the phase of an incident light beam can be modulated by the electro-optic effect when the angle of incidence of the light is not parallel to the plane of the smectic layers, or orthogonal to the plane containing the movement of the optic axis.